The present invention relates to golf clubs, more particularly to composite golf club shafts.
In hitting a golf ball with a golf club, deflection in the golf club shaft may adversely affect the angle at which the face of the golf club head strikes the golf ball. As the club is accelerated during the stroke, the face of the club is deflected from its static position by four principal forces acting on the club head: (1) the gross body acceleration of the club head in the plane of the swing (xe2x80x9clead-lagxe2x80x9d); (2) a transverse moment acting in a plane normal to the plane of the swing generated by the centrifugal force of the swing acting on the center of mass of the club head, which tends to rotate the club head toe down (xe2x80x9ctoe downxe2x80x9d); (3) a second centrifugal force acting on the center of mass of the club head, which tends to rotate the club face up (xe2x80x9cloft increasexe2x80x9d); and (4) a torsional moment caused along the shaft caused by the acceleration of the club head, which tends to open the face of the club head and increase slice.
Each of these forces are dynamically resisted by the elastic material properties of the shaft itself, which results in a shaft that is vibrating in a number of complex modes at the instant the golf ball is hit. These complex modes of vibration are especially disadvantageous if the deflection of the golf club shaft is inconsistent from stroke to stroke. On the other hand, not only is it impossible to completely eliminate the vibration modes from a golf shaft, it is quite necessary that the golf club shaft deflect during the stroke, for up to 10% of the energy delivered to the golf ball is energy stored in the bending of the shaft during the beginning of the stroke that is released during the time the ball is in contact with the club.
The prior art has addressed the need for shaft having a desirable lead-lag flexibility with a greater degree of control over unwanted vibration modes affecting club head contact angles. Virtually all prior art attempts, however, have concentrated on creating a discontinuity between the lower portion of the shaft and the grips. For example, U.S. Pat. No. 4,319,750 to Roy discloses a shaft which comprises a standard profile shaft having a discontinuity between the lower shaft and the butt portion intentionally introduced by mismatching the elasticity of the materials making up the two sections of the shaft. Similarly U.S. Pat. No. 5,439,219 to Vincent discloses a so-called xe2x80x9cBubblexe2x80x9d shaft comprising a discontinuous flexible zone interposed between a rigid butt section and the remainder of the shaft. U.S. Pat. No. 4,330,126 to Rumble discloses a metal golf club shaft having a discontinuity between a narrow but section having an outside diameter of 0.590 inches immediately adjacent a wider shaft section having a diameter of 0.620 inches immediately below the butt section. Bubble shafts and other shafts with intentional discontinuities are found disagreeable by many players, however, because such pronounced discontinuities in a golf club shaft produce an exaggerated and artificial feedback to the player""s hands, thereby reducing the subjective quality of the xe2x80x9cfeelxe2x80x9d of the shaft. Such intentional discontinuities also introduce additional degrees of freedom of vibration, in effect, introducing hinges into an already complex vibrational system. The additional hinges cause the shafts to vibrate in unusual and more complex modes than do shaft without such intentional discontinuities.
What is needed then, is a composite shaft with a high degree of flexure, without the artificial feel caused by pronounced discontinuities, and with a high degree of control over the amplitude of the unwanted vibration modes of the club head.
According to the present invention a composite golf club shaft is constructed having a reduced butt diameter of 0.400 to 0.560 inches in diameter, preferably from 0.450 to 0.550 and most preferably from 0.520 to 0.540 inches in diameter. In a xe2x80x9cstandard taperxe2x80x9d embodiment of the present invention, the shaft tapers without intentional discontinuities from the reduced-diameter butt section to a cylindrical tip portion having a standard tip diameter adapted to be attached to the hosel of a club head. In a standard xe2x80x9cparallel-taper-parallelxe2x80x9d embodiment of the present invention, the reduced-diameter cylindrical butt portion of the shaft continues without intentional discontinuity until it meets the tapered portion of the shaft, which in turn tapers until it meets the cylindrical tip portion of the shaft. By reducing the diameter of the butt portion of the shaft while maintaining the shaft free of substantial discontinuities, not only is the lead-lag flexure increased, with concomitant increase in distance, but the subjective feel of the club is improved with a concomitant increase in the player""s ability to control the club.